Case hardening of wood



March 2, 1965 c. H. VAN HARTESVELDT ETAL 3,171,167

CASE HARDENING OF woon Filed Feb. 2, 1.960 5 Sheets-Sheet 1- i &

APP FF 1 .PF 7 HHMWWWQ l. 'l- -IJ'll-l ll 6A1 W 621 A March 2, 1965 c. H. VAN HARTESVELDT ETAL 3,

CASE. HARDENING 0F woon Filed Feb. 2, 1960 5 Sheets-Sheet 2 Carroll H Mm Harfesueloz suoq a [Mk1 March 2, 1965 c. H. VAN HARTESVELDT ETAL 3,

CASE HARDENING OF WOOD Filed Feb. 2, 1960 5 Sheets-Sheet 3 5. a. 5- U 7; H33? /33 MWM W5 Carroll H. Van arzeszrelof Bud/y 0 Wk/ 5 Sheets-Sheet 4 HEIGHT OF DlsmMCE m D FOQCE ON WOOD March 2, 1965 c. H. VAN HARTESVELDT ETAL CASE HARDENING OF WOOD Filed Feb. 2, 1960 w rm.

2. dm n. nmwmkw so as STQNN OF ORIGINAL. 5:25

STEEL MANDEEL N m w M MANDREL Carroll H Van fiarleswlo'z United States Patent 3,171,167 CASE HARDENENG SE W001) Carroll H. Van Hartesveldt and Buddy D. Wahl, Toledo, Ohio, assignors, hy mesne assignments, to Hoover Ball and Bearing Company Filed Feb. 2, 1960, Ser. No. 6,189 3 iairns. (Cl. 20-91) The present invention relates to a mechanism and method for providing an improved wood product having a hardened compressed layer on the surface.

The wood product made in accordance with the mechanism and method has a surface layer which has been compressed beyond the elastic limit of the wood to form a hardened outer layer which is integral with the wood. Inasmuch as only the surface of the wood is compressed only a small volume of wood is sacrificed to obtain surface hardness. There are many environments where only surface resistance to wear and local impact is desired. In accordance with the present invention the original vol ume of the wood has not been substantially reduced and the wood is protected by an integral hardened surface.

An object of the invention is to provide an improved method and mechanism for obtaining a wood structure having an integral case hardened compressed outer layer.

Another object of the invention is to provide an improved mechanism and method for compressing only the outer layer of wood beyond its elastic limit and stabilizing the compressed layer to provide a permanent structure which can withstand moisture and not return to its original uncompressed state.

Another object of the invention is to provide an improved method and mechanism for providing a case hardened surface for wood impregnated with a plastic or resin sealant which may be covered with a decorative paper layer.

Another object of the invention is to provide an improved method and mechanism which can be used to compress the outer surface of wood beyond its elastic limit and provide a case hardened layer simply and inexpensively for production and manufacturing operations.

Another object of the invention is to provide an improved wood product having an outer compressed surface layer which is fully compressed to a specific gravity of 1.3-1.4 with the outer layer diminishing linearly in density away from the surface and the compressed outer layer supported by wood of natural density and full volume.

Other objects and advantages will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims and drawings, in which:

FIGURE 1 is a side elevational view of a mechanism, shown with portions in section for case hardening the outer surface of a wood workpiece in accordance with the method of the invention;

FIGURE 2 is a side elevational view similar to FIG- URE 1, showing additional details and illustrating additional steps of the method;

FIGURE 3 is a side elevational view similar to FIG- URE l and illustrating another form of the method;

FIGURE 4 is a side elevational view similar to FIG- URE l and illustrating still another form of the method;

FIGURE 5 is a side elevational view with portions shown in section, illustrating a simplified form of the mechanism in accordance with the present invention;

FIGURE 6 is an enlarged detailed sectional view illustrating in particular relative dimensions of a portion of the mechanism;

FIGURE 7 is a side elevational view, shown partially in section, illustrating the method being performed with rollers;

FIGURE 8 is a vertical sectional view taken through a 3,171,167 Patented Mar. 2, 1965 wood workpiece finished in accordance with the present invention;

FIGURE 9 is a graph showing the stress-strain relationship for a type of wood such as maple;

FIGURES 10 through 14 are vertical sectional views taken through a wood workpiece being subjected to action of a mandrel in accordance with the present invention, shown somewhat in schematic form for describing the dimensional relationships involved; and

FIGURE 15 is a graph showing the relationship between the specific gravity of the wood and the distance from the Wood surface.

As shown on the drawings:

The mechanisms illustrated in FIGURES 1 through 6 perform the method of the invention for forming a hardened compressed layer on the surface of a wood workpiece by applying a progressively increasing pressure to a limited area of the surface of the wood until the elastic limit is exceeded, such as by using a mandrel or shoe having an inclined compressing surface which engages a limited area of the wood and has a slope within critical limits to insure overcoming the compressive strength of the surface layer of the wood workpiece. The shoe applying its local pressure is progressively advanced across the wood workpiece to enlarge the compressed area until the surface is covered with a layer of compressed wood. The surface of the wood may be preheated to a temperature above 212 F. to the depth of crushing to reduce the force required and to minimize fiber damage during compression. The compressed layer is stabilized while held compressed by heating to a temperature of 300-360" F. to a depth sufiicient to reach all of the compressed wood. The wood is then preferably cooled to a temperature of 210 F. or less to prevent the release of steam and drying of the wood.

The method may include impregnation of the wood surface with a sealant such as resins or wax or a thermosetting resin which will then be cured in the stabilizing heating step. A surface preheat step may be incorporated with surface impregnation of resin before compressing by preheating in a range from 250 to 360 to dry the wood surface and B-stage the resin. An impregnated decorative paper may be laid down on the impregnated wood surface before compressing, with the paper impregnated with a B-stage resin.

As illustrated in FIGURE 1, the wood workpiece 16 in the form of an elongated fiat board is surface hardened on both sides in the mechanism illustrated. While the method may be practiced with hardening on one side only, the mechanism illustrated shows a simultaneous hardening of both sides which provides a finished wood workpiece hardened on both surfaces and avoids'complications of warpage of the wood workpiece which may occur when one surface alone is hardened. The workpiece 16 is moved relative to surface treating and compressing assemblies 17 and 17' by suitable means such as a bar 18 placed across the end of the workpiece and forced in a direction to move the workpiece ahead of it.

Compressing of the surface layers of the workpiece is accomplished by sloped compressing surfaces 19 and 19 for the respective sides of the wood, on mandrels or shoes 22 and 22'. To prevent the displacement of wood resin and fibers due to the drag of the shoe or mandrel, a flexible shim or sheet 20, 20 is positioned over the surfaces of the wood between the wood and the shoe compressing surfaces 19 and 19'. The shim may be of brass or of flexible steel preferably such as 1095 steel, and the shims may be anchored at the lead end of the wood to be drawn along therewith. A lubricant such as graphite or mineral oil is employed between the shim and the mandrel. Mineral oil is preferred and avoids the soiling effects of graphite.

Preheating of the wood surface is accomplished by preheating blocks 21, 21 which are positioned in advance of the compressing surface'19, 19' and are heated by suitableheating elements such as 21a, 210'. These impart sufficient heat to' the wood to heat it to a temperature above 212 F. to soften the surface fibers of the wood. a

The. preheat block has a smooth planar surface parallel to the wood and facing the wood for good heat transfer contact. The shims and 20' are sufiiciently thin to permit rapid heat transfer and to conform to the shape of the surface of the shoes. The size and temperature of the preheating blocks, and the size and temperature of the other heat transfer members are, of course, chosen relative to the speed at which the wood workpiece will be moved through the mechanism to attain the desired surface temperature of the wood.

The shoesor mandrels 22 and 22 are heated by heating elements such as 22:: and 22a to heat the wood to a temperature of 300 F. to 360 F.

Behind the mandrels' 22 and 22 are coolers or cooling blocks'23 and 23, illustrated as being hollow with a chamber 23a and 23a therein for a coolant fluid. The cooling blocks reduce the temperature of the wood to a. temperature of 210 F. or less, preventing the compressed moisture on the wood surface from flashing into steam to dry the surface and preventing the causing of blisters. An insulating wall 24 and 24' is provided between the mandrels 22 and 22' and the cooling blocks 23 and 23'.

The e'lements'of the assemblies, such as the preheating blocks 21" and 21', the mandrels 22 and 22', and the cooling blocks 23 and 23' are mounted as a unit and may be backed by plates or bars 25 and 25 which are supported by pressure bolts 26 that are threaded and receive nuts and function to hold the mandrels against the board with sufficient pressure to insure compressing the wood surface.

As illustrated in FIGURE 5, compression of the surface of a wood workpiece30 may be performed without the application of a stabilizing heat, for provision of wood to be used where it will not be subjected to moisture. A wood workpiece 30 is moved by suitable means, such as a bar 31 at the end, between mandrels 32 and 32' which have sloping compressing surfaces 33 and 33' and sheet material or shims 34 and 34' are positioned over the surface of the wood during engagement by the mandr'el's.

The finished wood product is illustrated in enlarged sectional view in FIGURE 8 at 40. The outer surfaces or zones 41 and 42 are compressed while the interior 43 of the wood is at its original density. As will be described later in connection with FIGURE 15 and with the analysis of the slope required for the mandrel, the density of the wood ranges from a completely compressed outer layer with a specific gravity of 1.31.4 at the outer skin surface and diminishes linearly through the depth of compression to where the wood is at its natural density.

As illustrated in FIGURE 2, a wood workpiece 45 is moved inbetween the assemblies 25 and 25' being forced forwardly by the bar 18 which has cables 46 attached to its ends drawn forwardly by a motor driven winch arrangement 47 that moves the wood workpiece 45 forwardly at a uniform speed. Movement at uniform speed is preferred to obtain an accompanying uniform heat application. Ahead of the wood workpiece is a perheating mechanism illustrated as being conveniently in the form of infra-red lamps 48 and 48. This preheating mechanism preheats the outer surface or skin of the wood from 200-375 F. to remove surface moisture. This prevents the acquisition of moisture by the wood from the air which would be trapped beneath the surface of the preheating blocks 21 and 21' and beneath the surface of the mandrels 22 and 22.

As illustrated in FIGURE 3, a wood workpiece 49 is sities.

forced between the surface compressing assemblies 25 and 25, but' the surfaces are first impregnated with a sealant such as a resin or wax or thermoplastic. The sealant is applied in a manner shown schematically by a tank at 50 and the coated Wood workpiece is passed through a drying oven where it is dried overnight or for an equivalent period at 110 F. with the sealant'penetrating the wood. The preheating lamps 48 and 48' then heat the impregnated wood surface to 250 F. to 360 F. to eliminate any moisture which has been gathered from the air and to B-stage the resin if resin is used. This heat is at'the outer skin surface ofthewood and notnecessarily to the full compression depth as excess heating to the full depth would excessively soften the wood fibers. Heating to compression depth is further accomplished by the preheat blocks 21 and 21'. The pressure of the mandrels will distribute the impregnantand the heat of the mandrels, in the case when a thermosetting resin is used, will cure the resin.

As illustrated in FIGURE 4, a wood workpiece 56 is moved between the surface hardening assemblies 25 and 25' after having been surface impregnated in the tank 50 and the drying oven 51 and after the surface has been preheated by the lamps 48 and 48. On the upper surface 58 ofthe wood a decorative resin impregnated sheet of paper is laid from a supply roll 57. The paper is impregnated with B-stage resin and the paper is joined to the resin coated upper surface of the workpiece; a protective coating of resin is formed on the outer surface of the workpiece.

In each of the arrangements, when the wood workpiece consists of a long board, means will be provided to guide the board and prevent it from bowing. Guide rollers may be provided, or the cable 46, FIGURE 2, may be provided with connections that engage and support the board at'its side, such as for example fittings which slidably clamp over the board and have eyelets slidable on the cable.

.While a mandrel or shoe having an inclined compress ing surface'is preferred, the method can be practiced by the use of a roller, such as illustrated in FIGURE 7. A wood workpiece 60 is moved relatively past a compressing roller 61. The roller must have a size within critical limits so as to engage a limited area of the wood surface and exceed the compressive strength of the'wood at the surface and the compressing roller will be small. To prevent bending of the roller it is backed by a first backing roller 62 which is additionally backed by supporting rollers'63 and 64'. Heating elements, not shown, may be provided to preheat the wood and to post-heat it after the surface has been compressed for stabilizing the harda forced pastthe mandrel with a thin shim 67 on the wood surface. A compression surface 68 compresses the wood and the mandrel has a trailing surface 69 parallel-to the wood surfaceand a leading surface 70 parallel to the uncompressed wood surface. The mandrel is provided with fairing or is rounded at locations 71 and 72 where the compressing surface joins the flat planar surfaces 69 and 70'. The slope or the relationship of L to H is critical as will become apparent from the following description and examples.

Various species and types of wood have varying den- When dried or seasoned for use, the moisture remaining is in the cell walls and air fills the cells. This allows the wood to be crushed to total solids at which point it is virtually impossible to compress it further. In the totally compressed condition virtually all species of wood reach a density of 1.34.4, irrespective of initial density.

A typical stress-strain curve for wood is shown in FIG- URE 9. The actual example plotted is edge-glued, vertioal grained maple with the compression force perpendicular to the grain and approximately parallel to the plates of summer wood. Compressing the wood in this manner is preferred for obtaining a good finished product.

It is the FIGURE 9 stress-strain relationship typical of all wood that enables the surface compression of wood to be accomplished by the process of the prsent invention. The critical design consideration is the slope and height of the compression step.

The general case is shown in FIGURE 10. As the wood is moved from light to left, as shown by the arrow, it will have an increasing downward force applied along the line XY. The wood which would have continued to a is compressed into a The wood which would have continued to b is compressed into [1 To have such compression occur, it is necessary that the distance the wood is compressed be large enough so that the elastic limit is exceeded. For a piece of maple 2" thick, with a modulus of elasticity of 2,000,000, and an elastic limit of 4,800 lbs. per sq. inch, a compression of .005" will initiate compression failure. Consequently, this effect will be disregarded in this analysis.

In considering the forces involved in the FIGURE 10 example, FIGURE 11 is useful.

With wood moving from right to left, F is the total force along the sloped portion of the mandrel. This force produces a stress which exceeds the elastic limit of the wood. Deeper into the wood, as at XZ, this force is spread over a larger area and at XW a still larger area. At successive depths the stress diminishes along these lines until the elastic limit of the wood is no longer exceeded. If this occurs at XW, then a case of compressed wood results at a depth equal to WY.

Because wood compresses at an increasingly greater stress after the elastic limit has been exceeded, the degree of compression increases from W to Y and reaches ultimate compression at Y.

General example Suppose that it is desired to case harden a piece of wood as shown in FIGURE 12. d is the dimension required for wood at its ultimate compressed density and d is compressed wood varying in density from the ultimate at the top of the layer to that for uncompressed wood at the bottom. Becaus this gradient is approximately linear, the height of the step in the mandrel H must equal Totally Compressed Density Initial Density 1)(d1+ 1/2012) H is, therefore, also the dimension of uncompressed wood which will be compressed into the wood below it. To determine the length of the slope required, the following calculation is necessarily based on FIGURE 13.

P is the force at YX. H, a and d are as described in FIGURE 13.

Along line ZX is the unit pressure necessary for the complete densification of the wood. Along WX is the unit pressure at the elastic limit of the wood.

4) Z (Stress at total compressionin 1 Figure 9, for maple, this is 7800 (5) ZX 8 (Stress at elastic limit-in Figure XW 2 9, this is 4800) Specific Example N0. 1'

e-owe)- From Formula 10.

Therefore, mandrel is With a mandrel constructed as above, an edge-glued maple board was run. The case hardened board Was then weighed and successive cuts were taken off the surface. The piece was weighed and measured after each cut and the density plotted against depth as shown in FIGURE 15.

The wood may be preheated before putting it through the mandrel to reduce the forces necessary. When this is done the mandrel design must be calculated using the wood properties at the temperature chosen.

When wood is physically compressed as described above, it takes a permanent set. However, if it is wet subsequently, it tends to return to its former state. By heating to 300-360 it can'be stabilized. In our process the compressed layer is stabilized by heating the mandrel and pushing the wood through slowly enough to first compress and then stabilize the Wood. Also, heating the wood by a portion of the mandrel prior to its passing by the compression step plasticizes the surface sufficiently to resist fiber fracture.

In order to apply a durable and waterproof surface to the compressed and stabilized case, we have found it desirable to soak the boards prior to processing in a 20% water solution of unpolymerized phenolic resin components. Then with the mandrel at 350 P. we not only case harden the Wood by compressing and stabilizing it, but we also create a cured phenolic resin surface. Because the phenolic solution does not penetrate far and because this surface material is compressed to about onehalf its original thickness, the layer is only a few thousandths of an inch thick. However, the phenolic resin layer can be built up by impregnating alphacellulose paper, curing it to a B-stage and placing it on top of the phenolic resin solution impregnated board When this combination is run through the process, a glossy layer of paper reinforced phenolic resin is married to the phenolic resin in the surface of the compressed wood.

A most important feature of the use of phenolic resin components described above is the sealing of the wood surface during the case hardening process. Without it, we have found that the compressed surface becomes extremely dry as indicated by its lateral shrinkage even though it would be expected .to bulge the top surface of the board at its unsupported sides.- Our best result is attained by using a radiant heater above the surface of the woodat its entrance to the mandrel. The intensity of heat is-set to bring the surface of the wood to 300 F.

for 20 seconds. This dries the phenolic resin solution and takes it' to a B-stage. This seals thewood surface before its subsequent travel under the mandrel.

In calculating the slope and height of a compression step there are several practical limitations to be considered. If the step is too'steep,-shearing' forces set up would strip off the top layerof wood. An incipient condition of this nature would break up the wood fibers being compressed at the surface by bending them too sharply'. Likewise, too steep and short a compression step would work the traveling shim past its elastic limit. These limitations can be alleviated somewhat by fairing the lead-in and exit contours of the step.

As the slope of the shoe is made flatter, the case hardening effect becomes less and less until the density gradi ent becomes so gradual that nothing significant is accomplished. This is shown in the following example:

Specific Example No. 2

Given a step in the mandrel of H=% and L=3", find the density distribution whenrunning a 1 maple board.

From FIGURE 14:

(14) From FIGURE 9, the density of wood along the Compression of Wood line is This equation is forthe straight line in FIGURE 9 between the points indicated at 7,800 and 4,800 using the coordinates-Stress-(vertical) and Density (horizontal). The density at a stress of 4,800'lbs. per sq. in. is 0.7 or that of undeformed wood. The density at 7,800 lbs. per sq. in. is 1.4 or thatof wood completely collapsed under compression. The slope of this straight line is The intercept at the horizontal axis (Stress=0) can be determined by noting that the density changes 0.7 for a stress change of 3000 lbs. per sq. in. Therefore, the stress must drop 4,800 or a change in density of Because 0.7 of this amount is necessary to move back to 0, 0.42 is the distance of the intercept to the left of the vertical axis. Therefore, the intercept-on the horizontal axis is0.42. Consequently, the equation for the straight line is Stress-0.42

1.4-'0.7' D- XStress 0.42

182 .909 S1 (from 7 7 D2 D D9= 1 16 s. .42 .a0s),s* .42

4800+2560 I (17) S 1335 -5500 lbs. per sq. 1n.

(18) D OO .420.86

D =0.74 (from 13) The variation in density from 0:86 to 0.74 in A3 of an inch is or .137 per inch. In Specific Example No. 1 and as verified by the sample run and reported as FIGURE 15, the variation in density is or 5.6 per inch. Therefore, this case hardening effect is or 41 times as much as the more gradual slope.

As used hereinbefore in the specific examples, the terms resin and B-stage resin makes specific reference to 20% aqueous phenol-formaldehyde resins. These resins may be converted to the B-stage by preliminary heating and ultimately thermoset during the passage between the mandrels. Resins of this type are particularly suitable in the practice of the invention'not only for the provision of a suitable resin impregnated wood surface but also for the provision of a suitable decorative paper surface over the compressed wood surface and adequately adhered thereto. On the other hand, it will be appreciated that satisfactory results may also be obtained using numerous other phenolic resins, such as the alkyl phenol-formaldehyde resins (e.g. p-tert.butylphenol-formaldehyde resins). In addition, other resinous aldehyde condensation products may be used to impregnate the wood surface and/or the paper covering, if such covering is used. Such other aldehyde condensation products include the well known ureaformaldehyde, melamine-formaldehyde, benzoguanamineformaldehyde (and other triazine-formaldehyde resins wherein the triazine has at least two unsubstituted amino groups), toluenesulfonamide-formaldehyde resins, etc. Preferably the resins of this class that are used are thermosetting resins which in their early stages of condensation are water-soluble so that they may be more readily applied to the wood and/ or paper and caused to impregnate the same. Moisture barriers may, however, be provided for the purpose of the instant invention by various thermoplastic resins which may be caused to impregnate the wood and/ or paper to at least a limited extent. In using such resins the wood material is adequately cooled before it is released from the mandrels.

The resins are usually used in conjunction with a polymerization catalyst and/ or accelerator; but such compositions are conventional and need not be described in greater detail herein.

Another type of resin which has been found to be particularly suitable for use with a paper covering (but which may also be used to impregnate the Wood itself) is a polymerizable unsaturated polyhydric alcohol-polycarboxylic acid polyester, which is prepared by reaction of one or more polyhydric alcohols and one or more polybasic acids. Such a resinous material is used to impregnate the Wood surface and/ or the paper in substantially the manner hereinbefore described in connection with phenolic resins. The proportion of polyhydric alcohols having more than two hydroxy groups, such as glycerol or pentaerythritol, and the proportion of polycarboxylic acids having more than two carboxy groups, such as citric acid, preferably is small so that in the production of the polyester there may be maximum esterification of the hydroxy and carboxy groups without attainment of excessive viscosity. Ordinarily it is desirable that the unsaturated polyester be polymerizable into an infusible or high melting point resin, so that the proportion of unsaturated components should be such that the polyester contains an average of more than one double bond per molecule; for example, there may be an average of eleven or more double bonds in every ten molecules of the polyester.

The present invention is applicable to all polymerizable unsaturated polyhydric alcohol-polycarboxylic acid polyesters. A typical example of such a polyester is a product prepared by the reaction of an unsaturated dicarboxylic acid such as maleic, fumaric, itaconic, citraconic or mesaconic acid with a dihydric alcohol such as any polymethylene glycol in the series from ethylene glycol to decamethylene glycol, propylene glycol, any butylene glycol, any polyethylene glycol in the series from diethylene glycol to nonaethylene glycol, dipropylene glycol, any glycerol rnonobasic acid monoester (in either the alpha or beta position), such as monoforrnin or monoacetin, any monoether or glycerol with a monohydric alcohol, such as monomet-hylin or monoethylene, or any dihydroxy alkane in which the hydroxy radicals are attached to carbon atoms that are primary or secondary or both,- in the series from dihydroxy butane to dihydroxy decane.

Part of the unsaturated dicarboxylic acid may be replaced by a saturated dicarboxylic acid, such as any normal acid in the series from oxalic acid and malonic acid to sebacic acid, or any benzene dicarboxylic, naphthalene, dicarboxylic or cyclohexane dicarboxylic acid, or diglycolic, dilactic or resorcinol diacetic acid. All of the unsaturated acid may be replaced by a saturated acid it a polyhydric alcohol is present whose molecule has two or three free hydroxy groups and consists of an ether of one or two molecules of allyl or methallyl alcohol with one molecule of a polyhydroxy compound such as glycerol, pentaglycerol, pentaerythritol, butan tetrol-1,2,3,4, a trihydroxy normal alkane having from four to five carbon atoms such as butantriol-1,2,3, or a monoalkyl ether ofpentaerythritol or butantetrol-1,2,3,4 in which the alkyl radical has from one to four carbon atoms and has from one to two hydrogen atoms attached to the same carbon atom as the ether linkage, such as the monomethyl or monoisobutyl ether of pentaerythritol.

In the preparation of the polymerizabl-e unsaturated polyester, any of the usual modifiers such as monobasic acids, monohydric alcohols and natural resin acids may be added. The larger the proportions of rnonobasic acids and monohydric alcohols, the lower is the average number of acid and alcohol residues in the resulting polyester molecules, and the lower is the viscosity of the polyester. On the other hand, the more nearly equal the molecular proportions of dibasic acid and dihydric alcohol, the greater is the average number of residues in the resulting polyester molecules, and the greater is the viscosity. The proportions of ingredients used are those proportions that produce a polymerizable polyester of the desired viscosity. Other properties of the polyester, such as solubility in various solvents, also may be varied by selecting various reacting ingredients and varying their proportions. The infusibility, hardness and inertness of the product obtained by polymerization of the polyester may be increased by varying the initial reacting ingredients to increase the average number of double bonds per molecule of the polymerizable polyester.

The point to which the reaction of the ingredients is carried in the preparation of the polymerizable polyester is simply that point at which the product has the desired consistency. The consistency or viscosity of the polyester varies directly with the average number of acid and alcohol residues in the molecule. For example, the average number of residues in the molecule of the polyester may vary from about three to about one hundred twenty.

The reaction is carried out at a temperature high enough and for a time long enough to secure the desired consistency. An elevated temperature preferably is employed to expedite the reaction, but during the preparation of the polyester, the temperature should not be so high nor the time of reaction so long as to cause substantial polymerization. There is less danger of premature polymerization if an inhibiting agent is added before the esterification is carried out.

The preparation of the unsaturated polyester preferably is carried out in an atmosphere of an inert gas such as carbon dioxide, nitrogen or the like, in order to prevent darkening or to make it possible to obtain a pale or colorless product. Bubbling the inert gas through the reacting ingredients is advantageous in that the gas serves the added functions of agitation and of expediting the removal of water formed by the reaction. It is desirable to exclude oxygen, which causes discoloration.

Polymerization of these materials usually is carried out at temperatures of about to about A solution comprising one or more polymerizable unsaturated polyesters and one or more polymerizable monomeric allyl esters hereinbefore described is particularly useful. Either the unsaturated polyester or the allyl ester or both may be partially polymerized before the ingredients are mixed. Allyl esters that are useful for the preparation of such a solution include diallyl phthalate, diallyl oxalate, diallyl diglycolate, triallyl citrate, carbonyl bis-(allyl lactate), maleyl bis-(allyl lactate), fumaryl bis-(allyl lactate), succinyl bis-(allyl lactate), adipyl bis-(a.llyl lactate), sebacyl bis-(allyl lactate), phthalyl bis-(allyl lactate), fumaryl bis-(allyl glycolate), carbonyl bis-(allyl glycolate), carbonyl bis-(allyl salicylate), tetra-(allyl glycolate) silicate, and tetra-(allyl lacfate) silicate. Such a solution, which usually contains about 20 to 80 percent of the allylester and about 70 to 80 percent of the polymerizable polyester, is particularly advantageous because the polyester has desirable physical properties and hardens very rapidly after the initial polymerization whereas the presence of the allyl ester causes the polymerized product to be much more water resistant and insoluble. Moreover, the combination of the polyester and the allyl ester usually polymerizes more rapidly than either of such substances alone. (The terms parts and percent, as used herein to refer to quantities of material, means parts and percent by weight.)

A similar solution may be prepared by dissolving the polyester, before use, in a polymerizable substance such as styrene, vinyl acetate, methyl methacrylate or methyl acrylate.

It will be understood that modifications and variations may be effected without departing from the spirit and scope of the novel concepts of the present invention.

We claim as our invention:

1. An article comprising afiat sheet of Wood having at least one planar outer surface and including a body portion of natural wood of substantially uncompressed original structure, said planar surface defined by a layer of the same natural wood of said body portion compressed beyond its elastic limit and being an integral continuation of the natural wood of said body portion, said layer being 12 coextensive with said body portion and providing a uniforni'thickn'es's zone of case-hardened wood of density greater than the body portion.

2. An article as claimed in, claim 1 wherein the compressed wood at said planar outer surface has a specific gravity of 1.3 to 1.4.

3. An article comprising a flat sheet of Wood having two parallel planar outer surfaces and including a body portion of natural Wood of substantially uncompressed original structure, each of said planar outer surfaces defined by a layer of the same natural wood of said body portion compressed beyond its elastic limit and being an integral continuation of the natural Wood of said body portion, said layer being coextensive with said body porf tion and providing a uniform thickness zone of case hardened wood of density greater than the body portion.

References Cited by the Examiner UNITED STATES PATENTS 514,847 2/94 Du Bois 144-2 575,973 1/97 Ma Lachlan 144-327 X 638,477 12/99 Scheid 144-2 1,465,383 8/23 Walsh et a1. 20-91 1,875,055 8/32 Loetscher. 1,952,664 3/34 Esselen 144-327 2,136,730 11/38 Sweetland 144-3096 X 2,343,740 3/44 Birmingham 20-91X 2,350,729 6/44 Crowet 20-91 2,362,269 11/44 Hall 20-91 2,586,308 2/52 Curtis 144-327 2,652,081 9/53 Curtis 144-327 2,886,074 5/59 Beitz 144-2 2,986,782 6/ 61 Elmend'orf 20-91 X FOREIGN PATENTS 64,289 9/92 Germany.

JACOB L. NACKENOFEPrimary Examiner.

MORRIS M. FRITZ, WILLIAM I. MUSHAKE, EARL J. WITMER, Examiners.

Patent No. 3,171,167 March 2, 1965 Carroll H. Van Hartesveldt et a1.

It is hereby certified that err ent requiring correction and that th corrected below.

or appears in the above numbered pate said Letters Patent should read as Column 5, line 49, for "Becaus" read Because column 5, lines 74 and 75, for

ZX d P W Tea 7W Signed and sealed this 3rd day of August 1965.

(SEAL) Attest:

ERNEST W. SWIDER' EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. AN ARTICLE COMPRISING A FLAT SHEET OF WOOD HAVING AT LEAST ONE PLANAR OUTER SURFACE AND INCLUDING A BODY PORTION OF NATURAL WOOD OF SUBSTANTIALLY UNCOMPRESSED ORIGINAL STRUCTURE, SAID PLANAR SURFACE DEFINED BY A LAYER OF THE SAME NATURAL WOOD OF SAID BODY PORTION COMPRESSED BEYOND ITS ELASTIC LIMIT AND BEING AN INTEGRAL CONTINUATION OF THE NATURAL WOOD SAID BODY PORTION, SAID LAYER BEING 